Open up the exhaust, 2-1/2" from the manifolds back; get a tuner or "tunes for life" deal and get rid of the stock tune.

Then replace the stock upper plenum, it is the weak point in the intake tract. 70 mm TB too though it's not as important as swapping out the plenum--more about that here.

Lower ratio (numerically higher) final drive gears will make the car pull stronger, 3.73:1 and 4.10:1 are the most popular picks. The tuner/tunes will let you correct speedometer for the new gears.

After that there's not a lot of "bang-for-the-buck" stuff to be bolted on. A lighter flywheel and UDPs will liven up hard acceleration, as will an aluminum driveshaft. However if you are not the sort to accelerate briskly you will never know they are there.

Aftermarket CAIs are a waste of money, the stock intake is a CAI and a pretty good one at that...

What mods do you already have? And like everyone else asked how much you want to spend and do you want simple bolt on's or enter motor work? My favorite mod would have to be my Borla stingers with bassani off road x-pipe it didn't give it a huge amount of power put it did give it some and the sound is amazing. You get performance/style/sound to me you can't beat that.